The tree is a chemical factory, she explained, and its products are part of a sophisticated survival strategy. The flowers contain terpene oils, which repel mammals that might feed on them. But the ash needs to attract pollinators, and so it has a powerful lactone fragrance that appeals to large butterflies and honeybees. The chemicals in the wafer ash, in turn, she said, provide chemical protection for the butterflies from birds, making them taste bitter.

Many similar unseen chemical relationships are going on in the world around us. “These are at the heart of connectivity in nature,” she said.

Ms. Beresford-Kroeger, 63, is a native of Ireland who has bachelor’s degrees in medical biochemistry and botany, and has worked as a Ph.D.-level researcher at the University of Ottawa school of medicine, where she published several papers on the chemistry of artificial blood. She calls herself a renegade scientist, however, because she tries to bring together aboriginal healing, Western medicine and botany to advocate an unusual role for trees.

She favors what she terms a bioplan, reforesting cities and rural areas with trees according to the medicinal, environmental, nutritional, pesticidal and herbicidal properties she claims for them, which she calls ecofunctions.

Wafer ash, for example, could be used in organic farming, she said, planted in hedgerows to attract butterflies away from crops. Black walnut and honey locusts could be planted along roads to absorb pollutants, she said.

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The butternut seed, endangered in North America. The fatty acids of the flower and developing testa can be used for abdominal surgery as a temporary adhesive.
Credit
Colin Rowe for The New York Times

“Her ideas are a rare, if not entirely new approach to natural history,” said Edward O. Wilson, a Harvard biologist who wrote the foreword for her 2003 book, “Arboretum America” (University of Michigan Press). “The science of selecting trees for different uses around the world has not been well studied.”

Miriam Rothschild, the British naturalist who died in 2005, wrote glowingly of Ms. Beresford-Kroeger’s idea of bioplanning and called it “one answer to ‘Silent Spring’ ” because it uses natural chemicals rather than synthetic ones.

But some of Ms. Beresford-Kroeger’s claims for the health effects of trees reach far outside the mainstream. Although some compounds found in trees do have medicinal properties and are the subject of research and treatment, she jumps beyond the evidence to say they also affect human health in their natural forms. The black walnut, for example, contains limonene, which is found in citrus fruit and elsewhere and has been shown to have anticancer effects in some studies of laboratory animals. Ms. Beresford-Kroeger has suggested, without evidence, that limonene inhaled in aerosol form by humans will help prevent cancer.

David Lemkay, the general manager of the Canadian Forestry Association, a nonprofit group that promotes the sustainable use of Canada’s forests, is familiar with her work. “She holds fast to the notion that if you are in the aura of a black walnut tree there’s a healing effect,” Mr. Lemkay said. “It needs more science to be able to say that.”

Memory Elvin-Lewis, a professor of botany at Washington University in St. Louis and co-author, with her husband, Walter H. Lewis, of “Medical Botany: Plants Affecting Human Health” (2003, John Wiley & Sons), said such a role for trees could be true. In India, she said, compounds from neem trees are said to have anti-inflammatory and antiviral properties and are planted around hospitals and sanitariums. “It’s not implausible,” Dr. Elvin-Lewis said; it simply hasn’t been studied.

On a more solid scientific footing, Ms. Beresford-Kroeger is also concerned about the fate of the Northern forests because of overharvest and the destruction of ecosystems. Federal scientists estimate more than 93 percent of old growth has been cut. As forests are fragmented, they dry out, losing wildlife and insect species. As a result, subtle relationships, the nerve system of biodiversity, are breaking down before they have been studied.

“In a walk through old growth forest, there are thousands if not millions of chemicals and their synergistic effects with one another,” she said. “What trees do chemically in the environment is something we’re only beginning to understand.”

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The early Siberian sour cherry can be grown in extreme circumstances.Credit
Colin Rowe for The New York Times

Trees also absorb pollutants from the ground, comb particulates from the air and house beneficial insects.

Some studies support a role for trees in human health. A recent study by researchers at Columbia found that children in neighborhoods that are tree-lined have asthma rates a quarter less than in neighborhoods without trees. The Center for Urban Forest Research estimates that each tree removes 1.5 pounds of pollutants from the air. Trees are also used to remove mercury and other pollutants from the ground, something called phytoremediation. And, of course, trees store carbon dioxide, which mitigates global warming.

Dr. Wilson, at Harvard, said that more research into the role of trees in the ecosystem was imperative and that it was alarming how little was known. “We need more research of this kind to use the things we have, such as trees, to their fullest,” he said.

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Both Dr. Wilson and Ms. Beresford-Kroeger proposed using stock from old-growth forests for planting new forest in the hopes of taking advantage of good genetics. “There’s an enormous difference between old-growth forests and tree plantations,” Dr. Wilson said.

Ms. Beresford-Kroeger is famous in Canadian horticulture circles for her sprawling gardens, which she maintains with her husband, Christian Kroeger, and are often open to the public. She has 60,000 daffodils, more than 100 rare hellebores from Turkey and Iran and extremely rare peonies from China that are dark brown with red leaves and smell like chocolate.

And she grows more than 100 types of trees, including rare fir trees and Siberian cherry trees, and disease-resistant chestnuts, elms and butternut.

Ms. Beresford-Kroeger recently completed the book “Arboretum Borealis” about the boreal forest in Canada, which cuts across the northern half of the country. Canadian officials have recently announced plans to preserve 55 million acres — roughly half. “Trees are a living miracle,” Ms. Beresford-Kroeger said. “Leaves can take in carbon dioxide and create oxygen. And all creatures must have oxygen.”